In photographic elements compounds which provide a photographically useful group, which is immobile, ballast stabilized or blocked at the active site thereof include positive forming compounds as illustrated in Belgian Pat. No. 810,195, U.S. Pat. Nos. 4,199,354, 3,980,479 and 4,139,379 and Japanese patent application (OPI) No. 130972/79. (The term "OPI" as used herein means an "unexamined published application".) These compounds are immobile or photographically inacitve but can release a mobile photographically useful group by an intra-molecular nucleophilic displacement reaction or intra-molecular electron transfer reaction.
Compounds having the aforesaid function have various merits and improving the characteristic and features of positive forming compounds has been desired to further increase the freedom of planning and the tolerance in the formation of photographic elements and the methods. In photographic elements before and after processing, the possibility of providing compounds with higher stability is desirable. In addition, the provision of more advantageous means for the control of the release of the photographically useful group or component is also desirable.
The present inventors studied reactions for cleaving a single bond in a compound after the compound has received an electron from a reducing substance. Various types of single bonds which may be relatively easily cleaved by reduction are known, including a carbon-sulfur bond and a carbon-halogen bond. However, such single bonds that can be easily cleaved by reduction but which are stable to acids, alkalis and heat are fairly limited. Further, such bonds that are stable to the aforesaid various conditions but which can be reduced by conventional organic reducing agents which are generally used in the field of photography to participate in the release of photographically useful groups are extremely limited. Some compounds are known which act in accordance with the above reaction, for example, as described in European Pat. No. 0045129A.sub.2 and West German (OLS) No. 3,008,588. However, it is difficult to conclude that these compounds sufficiently satisfy the desired characteristics of positive forming compounds as well as the necessary tolerance and freedom in the formation or planning of photographic elements, generally because of the chemical properties of the bond to be cleaved or some for reasons in the synthesis of the compounds.